1. Field of Invention
The present invention provides a method for protecting powder metallurgy alloy elements from oxidation and/or hydrolyzation during sintering and powder metal compositions formed in accordance with the method.
2. Description of Related Art
Traditionally, copper (Cu), nickel (Ni), and molybdenum (Mo) have been used as alloy elements in powder metallurgy part-making applications. The oxides of all three elements are easily reducible during sintering. Therefore the effectiveness of such alloy elements is generally what one would expect, and the resulting parts exhibit the properties one would expect. One recent problem with the use of these alloy elements, however, is that the cost of some of them, particularly nickel and molybdenum, has risen dramatically since 2003.
Another problem is that these alloy elements are not the most effective alloy elements. There are other alloy elements such as chromium (Cr), manganese (Mn), and silicon (Si) that could produce better results if there was a way in which one could use them as alloy elements in powder metallurgy. Some of the strongest and hardest steels, and the best electromagnetic steels, include these alloy elements.
The use of elemental chromium, manganese and silicon is problematic in conventional powder metallurgy. These elements are prone to oxidize and/or hydrolyze during sintering. When pre-alloyed with iron or steel powders, they are known to produce adverse affects in powder metallurgy processing such as poor compressibility. Furthermore, when they are combined with other elements or compounds (e.g., FeCr, FeMn, FeSi), they tend to be extremely abrasive, which adversely affects die wear. Elemental chromium has been successfully pre-alloyed and used in powder metallurgy, but it has to be run in a very dry furnace (−25° F. dew point) and is known to adversely affect the compressibility of the powder. Manganese, which would likely be the most effective alloying element, has not been pre-alloyed at useful levels due to poor compressibility and has not been admixed in elemental form effectively because it oxidizes and hydrolyzes during sintering. Silicon is also subject to oxidation. Oxides formed of both manganese and silicon are stable and hard to reduce in a normal sintering cycle for powdered metal.